Imaging apparatus, imaging system, its controlling method, and storage medium storing its program

ABSTRACT

An idling time period after applying a bias to a conversion element until a start of an accumulation of the conversion element for deriving an image and an accumulation period from the start of the accumulation to a termination of the accumulation are measured. An offset correction of the image is conducted by using a dark current accumulation charge quantity in the accumulation calculated based on the measured idling time period and accumulation period and stored dark current response characteristics. Thus, even just after applying the bias to the conversion element, the offset correction can be properly conducted. An imaging apparatus which can execute a good radiographing without increasing costs and a size even just after applying the bias to the conversion element is provided.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a division of application Ser. No.12/050,220, filed Mar. 18, 2008, the entire disclosure of which ishereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging apparatus, an imagingsystem, its controlling method, and its program and, more particularly,to an offset correcting technique regarding a photographed image.

2. Description of the Related Art

Hitherto, most of photographs have been film photographs (silver saltphotographs) obtained by using an optical camera and a silver salt film.In association with the development of a semiconductor technique, animaging apparatus such as a video camcorder which can photograph amotion image by using a solid-state imaging device using a silicon (Si)single crystalline sensor represented by a CCD type sensor or an MOStype sensor has been being developed. However, an image obtained by suchan imaging apparatus using the solid-state imaging device is inferior tothe film photograph in terms of the number of pixels and an S/N ratio.The film photograph is generally used in order to photograph a stillimage.

On the other hand, in recent years, demands for an image processing by acomputer, a preservation by an electronic file, a transmission of theimage by E-mail, and the like are increasing. An electronic imagingapparatus for outputting a photograph image, as a digital signal, whichis not inferior to the film photograph image is demanded. This is trueof not only general photographs but also inspection and medical fields.

For example, there is an X-ray radiograph as a photograph using aphotograph technique in the medical field. An X-ray generated from anX-ray source is irradiated to an affected part of a human body as anobject and the X-ray radiograph is used to discriminate the presence orabsence of a fracture of a bone or a tumor or the like based oninformation of a transmission of the X-ray. The X-ray radiograph iswidely used for a medical diagnosis for a long time. Ordinarily, theX-ray transmitted through the affected part enters a phosphor once, isconverted into visible light, and thereafter, is exposed to a silversalt film.

However, although the silver salt film has such advantages that asensitivity and a resolution are high, there is such disadvantages thatit is troublesome for development, it takes a time for preservation andmanagement, the film cannot be sent soon to a remote plate, and thelike. Therefore, an electron X-ray imaging apparatus for outputting thephotograph image, as a digital signal, which is not inferior to the filmphotograph image as mentioned above is demanded. This is true of notonly the medical field but also a non-destructive inspection field of aspecimen such as a structure or the like.

To meet such a demand, an imaging apparatus (Flat Panel Detector;hereinbelow, abbreviated to “FPD) using a large-scale sensor obtained bytwo-dimensionally arranging imaging devices having photoelectricconversion elements made of amorphous silicon hydride has been beingdeveloped. The FPD is realized by using such a principle that when anelectric field in the reverse direction is applied to the photoelectricconversion element, a photoelectric current according to a quantity ofincident light flows in a semiconductor layer.

According to the FPD, for example, a metal layer, an amorphous siliconlayer, or the like is deposited onto an insulating substrate whose oneside is equal to about 30 to 50 cm by using a sputtering apparatus, achemical vapor evaporation deposition apparatus (CVD apparatus), or thelike and the photoelectric conversion elements (photodiodes) and thinfilm transistors (hereinbelow, abbreviated to “TFTs”) are formed. Thus,for example, about (2000×2000) photoelectric conversion elements areformed, the electric field of a reverse bias is applied to them, and, atthe same time, charges flowing in the reverse direction of each of thephotoelectric conversion elements can be individually detected by theformed TFTs.

However, according to the FPD, a current called a dark current flowseven in a state where no light is irradiated, thereby causing anartifact in the image. Further, the dark current exerts an influence asa shot noise on the image and becomes one of factors which causedetecting ability, that is, a sensitivity (S/N ratio) of the wholeapparatus to be deteriorated. There is a case where such a deteriorationexercises an adverse influence on the medical diagnosis and the judgmentabout the inspection. For example, naturally, a problem occurs if one ofa focus and a defective part is overlooked due to the shot noise as acause. It is, therefore, important to reduce the dark current as much aspossible.

SUMMARY OF THE INVENTION

The dark current in the FPD has time-dependent response characteristicsas illustrated in FIG. 11. As illustrated in FIG. 11, the dark currentjust after a bias was applied to the photoelectric conversion element islargest and decreases gradually (is settled) with the elapse of time.The following two causes of it are considered.

One of them is that, generally, in the case of forming the photoelectricconversion element by an amorphous silicon semiconductor as a maincomponent material, defect levels are formed by one of a dangling bondin an amorphous semiconductor film and impurities mixed in a formingprocess. Those defect levels function as trapping levels. Even justafter the bias was applied or before it is applied, ones of electronsand holes have been trapped and, after the elapse of a time of a fewmilliseconds to tens of seconds, they are thermally excited to aconduction band or a valence band and a conduction current (darkcurrent) flows.

In the case of an MIS (Metal Insulator Semiconductor) type photoelectricconversion element, it is generally known that there are many trappinglevels particularly in an interface portion between a semiconductorlayer (I layer) and an injection blocking layer (for example, N layer).In the case of using the MIS type photoelectric conversion element of acrystalline type without using the amorphous semiconductor film, it isgenerally known that there are not as many trapping levels as those inthe case of using the amorphous semiconductor film although it dependson processing conditions and apparatus upon forming the element.However, in the interface portion between the semiconductor layer (Ilayer) and the injection blocking layer (for example, N layer), thereare many mismatched crystal lattices, the trapping level is not equal tozero, and there is a tendency of an output of the photoelectricconversion element illustrated in FIG. 11.

It is considered that the other one of the causes is concerned withcharacteristics of the injection blocking layer. For example, when theinjection blocking layer is made of N type amorphous silicon, ideally,no holes are injected into the semiconductor layer side. However,actually, particularly, in the case of amorphous, the N layer does notperfectly block the holes. The holes which have passed through the Nlayer and injected into the semiconductor layer (I layer) become thedark current. The holes are accumulated in the interface between thesemiconductor layer (I layer) and the insulating layer. An internalelectric field in the I layer is lightened together with theaccumulation of the holes. Since a quantity of holes injected into the Ilayer from the N layer decreases together with the lightening of theelectric field, the dark current is attenuated.

In a manner similar to the MIS type photoelectric conversion element,also in the case of a PIN type photoelectric conversion element havingamorphous silicon as a component material, it takes a predetermined timeuntil the dark current becomes stable after the bias was applied. It isconsidered that this is because of the defect levels existing in thefilm. Similarly, in the case of amorphous selenium, gallium arsenide,mercury iodide, lead iodide, or cadmium telluride which absorbs aradiation and directly converts into an electric signal, it likewisetakes a predetermined time until the dark current becomes stable.

As a method of eliminating such a response of the dark current whichdepends on the time as mentioned above, there is a method of alwayscontinuously applying the bias to the photoelectric conversion element.However, if the bias is continuously applied to the photoelectricconversion element, the number of defects in the semiconductor isincreased by the flowing current, the characteristics are graduallydeteriorated, and a phenomenon such as increase in dark current,decrease in photoelectric current, or the like appears. If the electricfield is continuously applied by the applying of the bias, not only thenumber of defects increases but also there is a case where it becomes acause of a shift of a threshold value of the TFT and a cause ofcorrosion of a metal which is used for wirings due to a movement of ionsand an electrolysis, resulting in a deterioration of the reliability ofthe whole apparatus. The deterioration of the reliability is undesirableupon manufacturing products of the medical apparatus and inspectingapparatus. For example, it is undesirable that the apparatus failsduring the diagnosis, treatment, or inspection which needs emergency.Therefore, it is necessary to design the FPD so that the photoelectricconversion element is made inoperative when the FPD is not used.

Unlike the photographing using the film, according to the FPD, since thephotograph image can be displayed onto a monitor and a diagnosis can bemade just after the photographing, it is expected to use the FPD in afield where the photographing and the diagnosis are performed in a shorttime as in emergency medical services. However, since the dark currentof the photoelectric conversion element as mentioned above has theresponse characteristics which depends on the time, just after the biaswas applied to the photoelectric conversion element, the dark current islarge and the artifact and noise occur, so that picture qualitydeteriorates.

Therefore, in the Official Gazette of U.S. Pat. No. 6,127,684, the X-rayradiographing is performed after the response of the dark current issettled. An offset image to which no X-ray is irradiated is radiographedafter or before the X-ray radiographing and a difference between theoffset image and the X-ray image obtained by the X-ray radiographing iscalculated, thereby removing the dark current component of the X-rayimage (hereinbelow, such a process is referred to as an offsetcorrection). According to the former method, since the operator has towait for a predetermined time after the bias was applied to thephotoelectric conversion element, there is such a problem that theapparatus cannot be used in case of emergency and an operability is low.According to the latter method, since the dark current is large justafter the bias was applied to the photoelectric conversion element,there is such a problem that even if the difference is calculated, thedark current component cannot be perfectly removed.

In the Official Gazette of U.S. Pat. No. 5,818,898, the dark current(noise quantity data) per unit time is stored into a memory.Accumulation noise quantity data is calculated based on an accumulationperiod upon radiographing measured by an accumulation period measuringcircuit and the noise quantity data per unit time, and the accumulationnoise quantity data is subtracted from the X-ray image, therebyconducting the offset correction. However, since the dark current hasthe response characteristics which depends on the time elapsed after thebias was applied to the photoelectric conversion element as mentionedabove, there is such a problem that the dark current component cannot beperfectly subtracted from the X-ray image.

In the Official Gazette of U.S. Pat. No. 6,965,111, the dark current isstabilized by irradiating light to the photoelectric conversion elementby using a light source such as LED, EL, or the like. However, the lightsource has to be equipped, resulting in an increase in costs and size ofthe FPD.

It is an object of the invention to provide an imaging apparatus inwhich even just after a bias was applied to a photoelectric conversionelement, a good radiographing can be performed without increasing costsand a size of the apparatus.

According to the invention, there is provided an imaging apparatuscomprising: a detection unit including a plurality of conversionelements arranged in an array on a substrate for converting an incidentradiation or incident light into an electric signal, to derive an imagebased on the electric signal; a memory unit for storing a dark currentresponse characteristics of the detection unit after applying a bias tothe conversion element; a first time period measuring unit for measuringa first time period from an applying of a bias to the conversion elementuntil a start of an accumulation of the conversion element for derivingthe image; a second time period measuring unit for measuring a secondtime period from a start of the accumulation until an end of theaccumulation; an accumulation charge quantity arithmetic operation unitfor calculating a dark current accumulation charge quantity included inthe accumulation based on the dark current response characteristics andthe first and second time periods; and an image processing unit forconducting an offset correction of the image derived based on the darkcurrent accumulation charge quantity.

According to the invention, there is provided an imaging systemcomprising: the image apparatus; and a radiation generating apparatusfor generating the radiation.

According to the invention, there is provided a controlling method of animaging apparatus having a detection unit including a plurality ofconversion elements arranged in an array on a substrate for convertingan incident radiation or incident light into an electric signal, toderive an image based on the electric signal and a memory unit forstoring a dark current response characteristics of the detection unitafter applying a bias to the conversion element, comprising steps of:measuring a first time period from an applying of a bias to theconversion element until a start of an accumulation of the conversionelement for deriving the image; measuring a second time period from astart of the accumulation until an end of the accumulation; calculatinga dark current accumulation charge quantity included in the accumulationbased on the dark current response characteristics and the first andsecond time periods; and conducting an offset correction of the imagederived based on the dark current accumulation charge quantitycalculated.

According to the invention, there is provided a storage medium forstoring a program for a controlling method of an imaging apparatushaving a detection unit including a plurality of conversion elementsarranged in an array on a substrate for converting an incident radiationor incident light into an electric signal, to derive an image based onthe electric signal and a memory unit for storing a dark currentresponse characteristics of the detection unit after applying a bias tothe conversion element, wherein the program controls a computer toexecute steps of: measuring a first time period from an applying of abias to the conversion element until a start of an accumulation of theconversion element for deriving the image; measuring a second timeperiod from a start of the accumulation until an end of theaccumulation; calculating a dark current accumulation charge quantityincluded in the accumulation based on the dark current responsecharacteristics and the first and second time periods; and conducting anoffset correction of the image derived based on the dark currentaccumulation charge quantity calculated.

According to the invention, the dark current accumulation chargequantity is calculated based on the idling time period from the applyingof the bias to the conversion element until the start of theradiographing for deriving the image, the accumulation period uponradiographing, and the dark current response characteristics. Since theoffset correction of the image is conducted by using the calculated darkcurrent accumulation charge quantity, even just after the bias wasapplied, the offset correction is properly conducted without increasingthe costs and size of the apparatus, and the good radiographing can beperformed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a schematic construction of an imagingsystem in the first embodiment.

FIG. 2 is a flowchart illustrating the processing operation of theimaging system in the first embodiment.

FIG. 3 is a timing chart illustrating drive timing of the imaging systemin the first embodiment.

FIG. 4 is a diagram illustrating an example in which sets eachconstructed by a photoelectric conversion element and a switchingelement have been arranged in a two-dimensional matrix form.

FIG. 5 is a timing chart illustrating drive timing in the constructionillustrated in FIG. 4.

FIG. 6 is a diagram illustrating a schematic construction of an imagingsystem in the second embodiment.

FIG. 7 is a timing chart illustrating drive timing of the imaging systemin the second embodiment.

FIG. 8 is a flowchart illustrating the processing operation of theimaging system in the second embodiment.

FIG. 9 is a timing chart illustrating drive timing of an imaging systemin the third embodiment.

FIG. 10 is a flowchart illustrating the processing operation of theimaging system in the third embodiment.

FIG. 11 is a diagram illustrating dark current response characteristicsof the photoelectric conversion element.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the invention will be described hereinbelowwith reference to the drawings.

First Embodiment

FIG. 1 is a schematic constructional diagram illustrating an imagingsystem in the first embodiment of the invention. In FIG. 1, aconstruction excluding an X-ray source 101 serving as a radiationgenerating apparatus and its control system corresponds to an imagingapparatus. The imaging system in the first embodiment is constructed bythe X-ray source 101, its control system, and the imaging apparatus. Inthe imaging apparatus in the first embodiment, a motion imagephotographing mode for executing a fluoroscopy or the like and a stillimage photographing mode for executing a radiography can be selectivelyfreely set.

The whole operation of the imaging system in the embodiment iscontrolled by a control unit 114A. A console unit 112 has a touch panelon a display 121, a mouse, a keyboard, a joystick, a footswitch, and thelike. In the imaging system in the embodiment, an operator 113 can makevarious types of settings such as radiographing conditions (still image,motion image, tube voltage, tube current, irradiation time period,etc.), radiographing timing, image processing conditions, object ID,processing method of a fetched image, and the like from the console unit112.

The radiographing conditions based on one of an instruction of theoperator 113 which is input through the console unit 112 and aninstruction of a radiation information system are instructed by thecontrol unit 114A to a radiographing control unit 115A for controlling aradiographing sequence and data is fetched. Based on the instructions,the radiographing control unit 115A drives the X-ray source 101 as aradiation source, a bedstead for radiographing (not shown), and aradiation detecting unit 103, fetches image data, and transfers to animage processing unit 110A. The image processing unit 110A executes animage processing designated by the operator 113 to the transferred imagedata, displays to the display 121, and at the same time, stores the rawdata obtained by executing fundamental image processings such as offsetcorrection, white correction, defect correction, and the like into amemory 111.

Further, based on the instruction of the operator 113, the control unit114A executes a re-image processing and a reproduction display regardingthe image data stored in the memory 111, a transfer and preservation ofthe image data into an apparatus on a network, a display onto thedisplay, a printing to a film, or the like.

Subsequently, the operation of the imaging system illustrated in FIG. 1will be sequentially described according to a flow of a signal. TheX-ray source 101 includes an X-ray tube and an X-ray diaphragm. TheX-ray tube is driven by a high-voltage generating power sourcecontrolled by the radiographing control unit 115A and irradiates anX-ray beam 122 as a radiation. The X-ray diaphragm is driven by theradiographing control unit 115A and shapes the X-ray beam 122 inassociation with a change in radiographing region so that theunnecessary X-ray irradiation is not executed.

The X-ray beam 122 irradiated from the X-ray source 101 is directed toan object 102 lying on the X-ray penetrating bedstead for radiographing.The radiographing bedstead is driven based on the instruction of theradiographing control unit 115A. The X-ray beam 122 irradiated to theobject 102 penetrates the object 102 and the radiographing bedstead and,thereafter, enters the radiation detecting unit 103.

The radiation detecting unit 103 has a grid (not shown), a phosphor 106,a photoelectric conversion element 104, a switching element 105, areading circuit 107, an A/D converter (ADC) 108, a drive circuit 109,and an X-ray exposure quantity monitor (not shown). The grid reduces aninfluence of an X-ray scatter caused after the X-ray beam 122 penetratedthe object 102. The grid is formed from an X-ray low-absorbing materialand an X-ray high-absorbing material and has a stripe structure of, forexample, Al (aluminum) and Pb (lead). At the time of the X-rayirradiation, the grid is vibrated based on the instruction of theradiographing control unit 115A so as not to cause a moire due to arelation of a lattice ratio of the radiation detecting unit 103 (in moredetail, photoelectric conversion element 104 arranged on the substrate)and the grid.

The phosphor 106 as a wavelength converter absorbs the X-ray whichpenetrated the object 102, excites the light emission center in thephosphor 106, and emits visible light. That is, the phosphor 106converts a wavelength of the incident X-ray. The visible light emittedfrom the phosphor 106 is irradiated onto a photosensitive surface of thephotoelectric conversion element 104 arranged on the insulatingsubstrate and photoelectrically converted. Further, thephotoelectrically converted signal charges are supplied to the readingcircuit 107 through the switching element 105 likewise arranged on theinsulating substrate and converted into a voltage signal by anintegrating amplifier of the reading circuit 107. The voltage signalconverted by the integrating amplifier of the reading circuit 107 isconverted from an analog signal into a digital signal by the ADC 108 andtransferred to the outside of the radiation detecting unit 103. Thedrive circuit 109 drives the photoelectric conversion element 104,switching element 105, and reading circuit 107 based on the control fromthe radiographing control unit 115A so that the reading operation of thesignal is executed in this manner.

As a photoelectric conversion element 104, for example, an MIS type orPIN type thin film photoelectric conversion element using amorphoussilicon hydride as a main component material, a PN photodiode usingmonosilicon (single crystalline silicon), or the like can be mentioned.As a switching element 105, a thin film transistor using amorphoussilicon, polysilicon (polycrystalline silicon), monosilicon, or the likeor a well-known MOS transistor can be used.

As a material of the insulating substrate, transparent glass in which analkali component is small is mainly used. As a material of the phosphor106, Gd₂O₂S:Tb, CsI:T1, or the like is used. The phosphor 106 is notlimited to such a material but can be made of, for example, one kind, asa main component material, selected from Gd₂O₂S, Gd₂O₃, CaWO₄, CdWO₄,CsI, and ZnS.

The photoelectric conversion element 104 can be also constructed so asto have a function for absorbing the X-ray without passing through thephosphor 106 and directly converting into the electric signal. Forexample, the photoelectric conversion element 104 may be also made ofone kind, as a main component material, selected from amorphousselenium, gallium arsenide, mercury iodide, lead iodide, or cadmiumtelluride.

The X-ray exposure quantity monitor monitors an X-ray transmissionquantity. The X-ray exposure quantity monitor can directly detect theX-ray by using a photosensitive element made of crystalline silicon orthe like or may detect the visible light which had penetrated thephotoelectric conversion element 104 and the switching element 105.Information detected by the X-ray exposure quantity monitor istransmitted to the radiographing control unit 115A. The radiographingcontrol unit 115A shuts off or adjusts the X-ray source 101 based on theinformation detected by the X-ray exposure quantity monitor. Althoughthe radiographing control unit 115A is provided out of the radiationdetecting unit 103 in the embodiment, the invention is not limited tosuch a construction but the radiographing control unit 115A may beprovided in the radiation detecting unit 103.

An X-ray room where the radiographing is executed and a control roomwhere the operator 113 executes the operation are different. The imagesignal from the radiation detecting unit 103 is transferred from theX-ray room for radiographing to the image processing unit 110A providedin the control room where the operator 113 executes the operation. Upontransferring, noises caused by the X-ray generation are large in theX-ray room and there is a case where the image data is not accuratelytransferred due to the noises. It is, therefore, necessary to raise anoise resistance of a transfer path. For example, it is desirable to usea transmission system having an error correcting function, adifferential signal transmission system represented by LVDS (Low VoltageDifferential Signaling), or a transfer path by an optical fiber.

The image processing unit 110A switches the display data based on aninstruction of the radiographing control unit 115A. As other functions,the image processing unit 110A executes a correction of the image data(offset correction, white correction, defect correction), a spacefiltering, a recursive process, and the like in a real-time manner and,further, can execute a gradation process, a scattered line correction,various types of spatial frequency processes, and the like. The imageprocessed in the image processing unit 110A is displayed on the display121. Simultaneously with the real-time image processing, the basic imagein which only the data correction has been conducted is stored in thememory 111. As a memory 111, a data storage device which can satisfy alarge capacity, a high speed, and a high reliability is desirable. Forexample, a hard disk array of RAID or the like is desirable.

The image data stored in the memory 111 is reconstructed so as tosatisfy a predetermined standard (for example, IS & C) and, thereafter,stored into an external storage device (not shown). The external storagedevice may be, for example, a magnetooptic disk, a hard disk in a fileserver in a LAN, or the like.

The imaging system in the embodiment can be also connected to the LANthrough a LAN board and has a structure having a data compatibility withan HIS. Naturally, a plurality of imaging systems can be connected tothe LAN and a monitor for displaying a motion image/still image, a fileserver for filing the image data, and the like are connected to the LAN.An image printer for outputting an image onto a film, an imageprocessing terminal for executing a complicated image processing andsupporting the diagnosis, and the are also connected. The imaging systemin the embodiment outputs the image data according to a predeterminedprotocol (for example, DICOM). In addition, a real-time remote diagnosisby a doctor can be performed upon X-ray radiographing by using themonitor connected to the LAN.

Subsequently, the offset correction in the imaging system in the firstembodiment will be described. The processing operation from the start ofthe operation until the image display will be sequentially describedhereinbelow along the flowchart illustrated in FIG. 2 with reference tothe schematic constructional diagram illustrated in FIG. 1, theflowchart illustrated in FIG. 2, and a timing chart illustrated in FIG.3. In the following description, a light output which is obtained byirradiating the radiation and executing the radiographing (radiationphotographing) (radiation image obtained by inputting the radiation) iscalled an X-ray frame and a dark output which is obtained by performingthe radiographing without irradiating the radiation is called an offsetframe.

First, a position of the object 102 and radiographing conditions such astube voltage, tube current, irradiation time period, and the like areset (S101). After that, the radiographing is started (S102). When theradiographing is started, the radiographing control unit 115A issues acommand to the drive circuit 109 in the radiation detecting unit 103.The drive circuit 109 which has received the command applies voltages toa bias wiring Vs, a gate wiring Vg, and a reference power source Vref ofthe reading circuit 107, respectively. By applying the voltages to thegate wiring Vg, the switching element 105 is turned on, a voltage(Vs−Vref) is applied to the photoelectric conversion element 104, and astate where the photoelectric conversion can be performed is obtained.

An idling illustrated in FIGS. 2 and 3 denotes a state where the biashas been applied to the photoelectric conversion element 104 asmentioned above. In the embodiment, whenever the operator 113 pressesthe X-ray irradiation switch, an operation mode can be immediatelyshifted to the reading operation. Upon idling, in order to reset theaccumulation charges due to the dark current generated by applying thebias to the photoelectric conversion element 104, pulses areperiodically applied to the gate wiring Vg, thereby turning on theswitching element 105.

As a start time of the idling operation, the radiographing control unit115A supplies time tis when the bias is applied to the photoelectricconversion element 104 of the radiation detecting unit 103 to an X-rayframe idling period measuring unit 117 and an offset frame idling periodmeasuring unit 124. The X-ray frame idling period measuring unit 117 andthe offset frame idling period measuring unit 124 stores the suppliedstart time tis of the idling operation (S103).

Subsequently, the X-ray irradiation switch is pressed by the operator113 at arbitrary timing. Thus, as a termination time of the idlingoperation, the radiographing control unit 115A supplies time (darkcurrent resetting time) txs when the switching element 105 is shiftedfrom the ON state to the OFF state just before the depression of theX-ray irradiation switch to the X-ray frame idling period measuring unit117. Further, the radiographing control unit 115A supplies the time txsas an accumulation start time of the X-ray frame to an X-ray frameaccumulation period measuring unit 120. The X-ray frame idling periodmeasuring unit 117 stores the supplied termination time txs of theidling operation and the X-ray frame accumulation period measuring unit120 stores the supplied accumulation start time txs of the X-ray frame(S104).

The X-ray frame idling period measuring unit 117 calculates an idlingtime period Txi (=txs−tis) of the X-ray frame based on the start timetis and the termination time txs of the idling operation and outputs.The X-ray frame idling period measuring unit 117 corresponds to a firsttime period measuring unit. Since the arithmetic operating process bythe X-ray frame idling period measuring unit 117 is executed in parallelwith the radiographing process, even during the arithmetic operatingprocess, the X-ray is irradiated and, after the termination of theirradiation, the radiation detecting unit 103 shifts the operation modeto the reading operation of the X-ray frame.

Upon reading the X-ray frame, by turning on the switching element 105 byapplying the voltage to the gate wiring Vg, the charges in thephotoelectric conversion element 104 are taken out by the readingcircuit 107, converted into the digital signal by the ADC 108, andtransferred to the image processing unit 110A. Thus, the X-ray image(radiation image) obtained in the X-ray frame is transferred to theimage processing unit 110A (S105).

In the reading of the X-ray frame, a quantity of charges output from thephotoelectric conversion element 104 is equal to the sum of a darkcurrent accumulation charge quantity Qwx and an X-ray charge quantity Qxas shown in a photoelectric conversion element current in the timingchart of FIG. 3. The dark current accumulation charge quantity Qwxdenotes a quantity of charges which are accumulated by the dark currentaccording to the dark current response characteristics of thephotoelectric conversion element 104. The X-ray charge quantity Qxdenotes a quantity of charges obtained by the photoelectric conversionby the X-ray irradiation.

Subsequently, since the switching element 105 is shifted from the ONstate to the OFF state, the transfer of the charges accumulated in thephotoelectric conversion element 104 is terminated and the reading ofthe X-ray frame is terminated. At this time, as a reading terminationtime of the X-ray frame, the radiographing control unit 115A suppliestime txe when the switching element 105 is shifted from the ON state tothe OFF state to the X-ray frame accumulation period measuring unit 120.The X-ray frame accumulation period measuring unit 120 stores thesupplied reading termination time txe of the X-ray frame. The X-rayframe accumulation period measuring unit 120 calculates an accumulationperiod Tx (=txe−txs) of the X-ray frame based on the accumulation starttime txs and the reading termination time txe of the X-ray frame andoutputs (S106). The X-ray frame accumulation period measuring unit 120corresponds to a second time period measuring unit.

After the termination of the reading of the X-ray frame, an arithmeticoperation unit 118 of the X-ray frame accumulation charge quantitycalculates and predicts the dark current accumulation charge quantityQwx of the X-ray frame by using the idling time period Txi and theaccumulation period Tx of the X-ray frame and the dark current responsecharacteristics stored in a dark current response characteristic memory119. The dark current response characteristic memory 119 corresponds toa memory unit. Upon calculation of the dark current accumulation chargequantity Qwx of the X-ray frame, it is obtained by integrating a valueobtained by calculating dark current response characteristics f(t) asshown in the following equation (1) with respect to a range from Txi to(Txi+Tx).Qwx=∫ _(Txi) ^(Txi+Tx) f(t)dt  (1)

In this instance, the smaller a difference between the actual darkcurrent accumulation charge quantity of the X-ray frame and thecalculated dark current accumulation charge quantity of the X-ray frameis, the image correction of the smaller artifact can be realized by theoffset correction.

As for the dark current response characteristics f(t), for example,coefficients A and B approximated by an exponential function arepreliminarily stored into the memory as shown in the following equation(2) and a calculation regarding the dark current accumulation chargequantity Qwx of the X-ray frame is executed.f(t)=Aexp(−Bt)  (2)

Although the dark current response characteristics have beenapproximated by using the exponential function in the embodiment,another function may be used so long as it can approximate the responsecharacteristics. When obtaining the dark current responsecharacteristics, there is a method of obtaining them upon shipping fromthe factory, a method of periodically updating them at an equippinglocation, or the like. However, if the response characteristics justbefore the radiographing are used, since there is no influence of theaging conversion in the photoelectric conversion element, the offsetcorrection of higher precision can be conducted.

Subsequently, the offset frame is radiographed. In the case ofcontinuously radiographing the offset frame after the termination of thereading of the X-ray frame, accumulation start time tws of the offsetframe becomes reading termination time of the X-ray frame. In the casewhere a time period is provided in order to reduce an influence of anx-ray image lag regarding the X-ray frame and the offset frame isradiographed, the accumulation start time tws of the offset framebecomes time when the switching element 105 is shifted from the ON stateto the OFF state just before the offset frame. In the embodiment, it isassumed that the offset frame is radiographed subsequently to thetermination of the reading of the X-ray frame.

As accumulation start time of the offset frame, the radiographingcontrol unit 115A supplies the reading termination time tws of the X-rayframe to the offset frame idling period measuring unit 124. The offsetframe idling period measuring unit 124 stores the supplied accumulationstart time of the offset frame (S107).

The offset frame idling period measuring unit 124 calculates an idlingtime period Twi of the offset frame based on the time (start time of theidling operation) tis when the bias is applied to the photoelectricconversion element 104 and the accumulation start time tws of the offsetframe.

Subsequently, an arithmetic operation unit 116 of the offset frameaccumulation period calculates an accumulation period Tw of the offsetframe according to the following equation (3) by using the dark currentaccumulation charge quantity Qwx of the X-ray frame calculated asmentioned above, the idling time period Twi, and the dark currentresponse characteristics f(t) (S108).Qwx=∫ _(Twi) ^(Twi+Tw) f(t)dt  (3)

The offset frame accumulation period arithmetic operation unit 116outputs the calculated accumulation period Tw of the offset frame to theradiographing control unit 115A. After the elapse of the accumulationperiod Tw of the offset frame from the accumulation start time tws ofthe offset frame, the radiographing control unit 115A issues a readingstart command of the offset frame to the radiation detecting unit 103.The radiation detecting unit 103 which has received the command readsout the offset frame (S109, S110). Since the reading of the offset frameis executed in a manner similar to that of the X-ray frame, itsdescription is omitted.

After the termination of the reading of the offset frame, the read-outoffset correction data is transferred to the image processing unit 110A.The image processing unit 110A executes a subtracting process (offsetcorrection) of (X-ray image)−(offset correction data), further executesa necessary image processing such as gain correction or the like (S111),and allows the corrected image to be displayed onto the display 121(S112).

In this manner, the dark current accumulation charge quantity Qwx of theX-ray frame is calculated and predicted by using the idling time periodTxi of the X-ray frame, the accumulation period Tx of the X-ray frame,and the dark current response characteristics f(t). An accumulationperiod of the offset frame to obtain the charges, as offset correctiondata, of the same quantity as the calculated dark current accumulationcharge quantity Qwx of the X-ray frame is determined. The offsetcorrection of the X-ray image is conducted by the obtained offsetcorrection data. Thus, an increase in size of the apparatus due to theaddition of the new light source or the like is eliminated and even inthe case of the dark current response characteristics in which a changejust after applying the bias to the photoelectric conversion element 104is large, the offset correction can be properly conducted and the goodradiographing can be performed. By alternately repeating such aradiographing (radiation photographing) of the X-ray frame and theradiographing of the offset frame, the continuous radiographing ofmotion images can be also executed.

Although the description has been made above on the assumption that onephotoelectric conversion element 104 and one switching element 105 areprovided, actually, a plurality of photoelectric conversion elements 104and a plurality of switching elements 105 are arranged on the insulatingsubstrate in a one-dimensional or two-dimensional array shape.

FIG. 4 illustrates an example in which sets (pixels) each constructed bythe photoelectric conversion element and the switching element arearranged in a two-dimensional matrix shape of 3×3. Although the matrixof (3×3) pixels has been illustrated for simplicity of description, thenumber of pixels can be arbitrarily set. The invention can be alsoapplied to an area sensor of, for example, (2000×2000) pixels or more.

In FIG. 4, Sm-n denotes a photoelectric conversion element; Tm-n aswitching element (TFT transistor); 123 a shift register forsequentially turning on the switching elements; and Vgm a gate wiringfor transferring pulses for driving the switching elements. An denotesan amplifier (arithmetic operation amplifier) for reading out chargesaccumulated in the photoelectric conversion element Sm-n; Cfn anintegration capacitor for integrating the signal charges when thecharges are read out of the photoelectric conversion element Sm-n; Mn asignal wiring for transferring the signal charges; Vs the bias wiringfor applying the bias to the photoelectric conversion element Sm-n; andVref the reference power source of the reading circuit constructed bythe amplifier An and the integration capacitor Cfn. Each of m and ndenotes a suffix (m=natural number of 1 to 3, n=natural number of 1 to3; this is true of the following description).

One end of the photoelectric conversion element Sm-n of the mth row andthe nth column is connected to the bias wiring Vs and the other end isconnected to the signal wiring Mn of the nth column through theswitching element Tm-n of the mth row and the nth column. Controlterminals (gates of the transistors) of the switching elements Tm-1 toTm-3 of the mth row are connected to the gate wiring Vgm of the mth row.One input of the amplifier An of the nth column is connected to thereference power source Vref and the other input is connected to thesignal wiring Mn of the nth column. The integration capacitor Cfn isconnected in parallel between the other input and an output of theamplifier An.

FIG. 5 is a timing chart illustrating drive timing in the case where thesets of the photoelectric conversion elements and the switching elementsin the radiation detecting unit 103 are constructed as illustrated inFIG. 4. In the case of the construction illustrated in FIG. 4, since ithas the matrix structure of (3×3) pixels, the idling time period of theX-ray frame and the accumulation period of the X-ray frame are measuredevery gate wiring and the accumulation period of the offset frame ismeasured every gate wiring.

First, when the radiographing is started, the radiographing control unit115A issues a command to the drive circuit 109 in the radiationdetecting unit 103. The drive circuit 109 which has received the commandapplies the voltages to the bias wiring Vs and the reference powersource Vref of the reading circuit, respectively. The radiographingcontrol unit 115A sequentially drives the gate wirings Vg1, Vg2, and Vg3by the shift register 123, thereby sequentially turning on/off theswitching elements T1-n, T2-n, and T3-n. By applying the bias andturning on the switching element Tm-n, the voltage (Vs−Vref) is appliedto the photoelectric conversion element Sm-n and the dark current startsto flow. By applying the voltage to the gate wiring Vgm, the chargesaccumulated by the dark current by the photoelectric conversion elementSm-n can be reset.

As a start time of the idling operation, the radiographing control unit115A supplies time tism when the bias has been applied to thephotoelectric conversion element Sm-n to the X-ray frame idling periodmeasuring unit 117 and the offset frame idling period measuring unit124. The X-ray frame idling period measuring unit 117 and the offsetframe idling period measuring unit 124 stores the supplied start timetism of the idling operation into the internal memory, respectively.

Subsequently, the X-ray irradiation switch is pressed by the operator.As termination time of the idling operation, the radiographing controlunit 115A supplies time txsm when the switching element Tm-n is shiftedfrom the ON state to the OFF state just before the depression of theX-ray irradiation switch to the X-ray frame idling period measuring unit117. The X-ray frame idling period measuring unit 117 stores thesupplied termination time txsm of the idling operation and calculates anidling time period Txim (=txsm−tism) of the X-ray frame every gatewiring Vgm.

As accumulation start time of the X-ray frame, the radiographing controlunit 115A supplies the time txsm when the switching element Tm-n isshifted from the ON state to the OFF state just before the depression ofthe X-ray irradiation switch to the X-ray frame accumulation periodmeasuring unit 120. The X-ray frame accumulation period measuring unit120 stores the supplied accumulation start time txsm of the X-ray frameinto the internal memory.

Subsequently, the X-ray is irradiated, the photoelectric conversion isexecuted in the photoelectric conversion element Sm-n, and the chargesare generated. Thus, the radiographing control unit 115A sequentiallydrives the gate wirings Vgm and the charges accumulated in thephotoelectric conversion element Sm-n are read out as a voltage signalby the amplifier An. The read-out voltage signal is A/D converted andtransferred as an X-ray image to the image processing unit 110A. Asreading termination time of the X-ray frame, the radiographing controlunit 115A supplies time txem when the switching element Tm-n is shiftedfrom the ON state to the OFF state to the X-ray frame accumulationperiod measuring unit 120. The X-ray frame accumulation period measuringunit 120 stores the supplied reading termination time txem of the X-rayframe and calculates an accumulation period Txm (=txem−txsm) of theX-ray frame every gate wiring Vgm.

Subsequently, the arithmetic operation unit 118 of the X-ray frameaccumulation charge quantity reads out the idling time period Txim ofthe X-ray frame from the dark current response characteristic memory119, the accumulation period Txm, and the dark current responsecharacteristics of every gate wiring and calculates and predicts a darkcurrent accumulation charge quantity of the X-ray frame.

Subsequently, as accumulation start time of the offset frame, theradiographing control unit 115A supplies time twsm to the offset frameidling period measuring unit 124. The offset frame idling periodmeasuring unit 124 stores the supplied accumulation start time twsm ofthe offset frame and calculates an idling time period Twim (=twsm−tism)of the offset frame every gate wiring Vgm.

Further, the arithmetic operation unit 116 of the offset frameaccumulation period calculates the accumulation period Twm of the offsetframe by using the dark current accumulation charge quantity of theX-ray frame, the idling time period Twim, and the dark current responsecharacteristics every gate wiring Vgm. The offset frame accumulationperiod arithmetic operation unit 116 outputs the calculated accumulationperiod Twm of the offset frame to the radiographing control unit 115A.

In a manner similar to the example illustrated in FIGS. 1 to 3, thereading of the offset frame is executed based on an instruction of theradiographing control unit 115A hereinbelow and the offset correction orthe like is executed in the image processing unit 110A.

In the case of the area sensor having a plurality of pixels, as a methodof deciding the accumulation period of the offset frame, there is amethod of deciding it by using a mean value of all of the pixels, amethod of deciding it every gate wiring as in the embodiment, or amethod of deciding it on a pixel unit basis. Since the dark currentresponse characteristics of the photoelectric conversion element differevery pixel depending on a variation upon manufacturing, it is desirableto decide the accumulation period of the offset frame on a pixel unitbasis. By this method, the accumulation charge quantity of the X-rayframe due to the dark current and that of the offset frame can beperfectly made identical. However, since there is also a case where thememory capacity, processing time period, and reading time periodincrease, it is sufficient to control the accumulation period of theoffset frame on a proper unit basis.

Second Embodiment

The second embodiment of the invention will now be described. In theforegoing first embodiment, after the X-ray frame was radiographed(radiation photograph), the offset frame is radiographed. The followingcase is considered: that is, if the offset frame is radiographed afterradiographing the X-ray frame, an image lag occurs by an influenceexerted by irradiating the light to the photoelectric conversionelement, an image lag component is mixed into the offset frame, and anartifact occurs in the image obtained after the offset correction.

In the second embodiment, therefore, the offset frame is radiographedbefore radiographing (radiation photograph) the X-ray frame. Thus, it ispossible to certainly prevent the influence of the image lag due to theradiographing of the X-ray frame from being exercised on the offsetframe, it is possible to certainly prevent the artifact from occurringin the image obtained after completion of the offset correction, and thegood image can be obtained without extending the radiographing timeperiod.

FIG. 6 is a schematic constructional diagram of an imaging system in thesecond embodiment. In FIG. 6, component elements having substantiallythe same functions as those of the component elements illustrated inFIG. 1 are designated by the same reference numerals and theiroverlapped description is omitted here. Also in FIG. 6, a constructionexcluding the X-ray source 101 serving as a radiation generatingapparatus and its control system corresponds to an imaging apparatus.The imaging system is constructed by the X-ray source 101, its controlsystem, and the imaging apparatus.

The whole operation of the imaging system in the embodiment iscontrolled by a control unit 114B. The control unit 114B corresponds tothe control unit 114A in the first embodiment although there is adifferent portion in an internal construction. A radiographing controlunit 115B corresponds to the radiographing control unit 115A in thefirst embodiment. An image processing unit 110B corresponds to the imageprocessing unit 110A in the first embodiment and the processingoperation regarding the offset correction differs from that of the imageprocessing unit 110A.

Although the second embodiment will be described on the assumption thatone photoelectric conversion element 104 and one switching element 105are provided, a plurality of photoelectric conversion elements 104 and aplurality of switching elements 105 may be arranged on the insulatingsubstrate in a one-dimensional or two-dimensional array shape.

The offset correction in the imaging system in the second embodimentwill be described. The processing operation from the start of theoperation until the image display will be sequentially describedhereinbelow along a flowchart illustrated in FIG. 8 with reference tothe schematic constructional diagram illustrated in FIG. 6, a timingchart illustrated in FIG. 7, and the flowchart illustrated in FIG. 8.

First, the position of the object 102 and radiographing conditions areset (S201). After that, the radiographing is started (S202). When theradiographing is started, in a manner similar to the first embodiment,the voltages are applied to the bias wiring Vs, the gate wiring Vg, andthe reference power source Vref of the reading circuit 107 by the drivecircuit 109 based on an instruction from the radiographing control unit115B, respectively. The start time tis of the idling operation when thebias has been applied to the photoelectric conversion element 104 issupplied from the radiographing control unit 115B to the X-ray frameidling period measuring unit 117 and the offset frame idling periodmeasuring unit 124 and stored (S203).

When the bias is applied to the photoelectric conversion element 104,the dark current starts to flow in the photoelectric conversion element104. Therefore, the voltage is periodically applied to the gate wiringVg before the X-ray irradiation switch is pressed by the operator 113.Thus, the accumulated charges by the dark current are periodically resetand the dark current (shot noises) accumulated in the offset frame canbe reduced.

Subsequently, the X-ray irradiation switch is pressed by the operator113 (S204). Thus, as termination time of the idling operation, theradiographing control unit 115B supplies the time tws when the switchingelement 105 has been turned off just before the X-ray irradiation switchis pressed to the offset frame idling period measuring unit 124.Further, as accumulation start time of the offset frame, theradiographing control unit 115B supplies the time tws to a measuringunit 126 of the offset frame accumulation period. The offset frameidling period measuring unit 124 stores the supplied termination timetws of the idling operation and the offset frame accumulation periodmeasuring unit 126 stores the supplied accumulation start time tws ofthe offset frame (S205).

The offset frame idling period measuring unit 124 calculates the idlingtime period Twi (=tws−tis) of the offset frame based on the start timetis of the idling operation and the accumulation start time tws of theoffset frame and outputs.

After that, after the elapse of an arbitrary accumulation period, thevoltage is applied to the gate wiring Vg, the switching element 105 isturned on, and the reading of the offset frame is started (S206). Whenthe switching element 105 is turned off and the reading of the offsetframe is terminated, as reading termination time of the offset frame,the radiographing control unit 115B supplies time twe when the readinghas been terminated to the offset frame accumulation period measuringunit 126 (S207).

The offset frame accumulation period measuring unit 126 stores thesupplied reading termination time twe of the offset frame. The offsetframe accumulation period measuring unit 126 calculates the accumulationperiod Tw (=twe−tws) of the offset frame based on the accumulation starttime tws of the offset frame and the time twe when the reading has beenterminated and outputs. The data read out by the reading operation ofthe offset frame is stored as offset data.

An arithmetic operation unit 127 of the offset frame accumulation chargequantity calculates a dark current accumulation charge quantity Qw ofthe offset frame based on the idling time period Twi and theaccumulation period Tw of the offset frame and the dark current responsecharacteristics stored in the dark current response characteristicmemory 119.

As accumulation start time of the X-ray frame, the radiographing controlunit 115B supplies the time txs when the switching element 105 is turnedoff and the reading of the offset frame has been terminated to the X-rayframe idling period measuring unit 117 and the X-ray frame accumulationperiod measuring unit 120. The X-ray frame idling period measuring unit117 stores the supplied accumulation start time txs of the X-ray frameand calculates the idling time period Txi (=txs−tis) of the X-ray framebased on the start time tis of the idling operation and the accumulationstart time txs of the X-ray frame and outputs. The X-ray frameaccumulation period measuring unit 120 stores the supplied accumulationstart time txs of the X-ray frame. The X-ray frame idling periodmeasuring unit 117 corresponds to the first time period measuring unit.The X-ray frame accumulation period measuring unit 120 corresponds tothe second time period measuring unit.

After the reading of the offset frame was terminated, the X-ray isirradiated and the reading of the X-ray frame is started (S208, S209).Upon reading of the X-ray frame, the quantity of charges which areoutput from the photoelectric conversion element 104 is equal to the sumof the dark current accumulation charge quantity Qwx of the chargesaccumulated by the dark current according to the dark current responsecharacteristics and the X-ray charge quantity Qx of the charges obtainedby the photoelectric conversion due to the irradiation of the X-ray.

When the reading of the X-ray frame is terminated, as a readingtermination time of the X-ray frame, the radiographing control unit 115Bsupplies the time txe when the reading has been terminated to the X-rayframe accumulation period measuring unit 120 (S210). The X-ray frameaccumulation period measuring unit 120 stores the supplied readingtermination time txe of the X-ray frame. The X-ray frame accumulationperiod measuring unit 120 calculates the accumulation period Tx of theX-ray frame based on the accumulation start time txs and the readingtermination time txe of the X-ray frame and outputs.

Further, the arithmetic operation unit 118 of the X-ray frameaccumulation charge quantity calculates the dark current accumulationcharge quantity Qwx of the X-ray frame by the equation (1) by using theidling time period Txi and the accumulation period Tx of the X-ray frameand the dark current response characteristics stored in the dark currentresponse characteristic memory 119.

Subsequently, the image processing unit 110B multiplies the offset imageas offset data by (Qwx/Qw) by using the dark current accumulation chargequantity Qw of the offset frame and the dark current accumulation chargequantity Qwx of the X-ray frame, thereby conducting the correctionregarding the dark current component. The image processing unit 110Bconducts the offset correction of the X-ray image by using the darkcurrent corrected offset correction data and subsequently executes thenecessary various types of image processing such as gain correction andthe like (S211), thereby allowing the corrected image to be displayedonto the display 121 (S212).

As described above, the dark current accumulation charges can be alsoequalized by the image processing. Even just after applying the bias tothe photoelectric conversion element 104, the proper offset correctionaccording to the dark current response characteristics can be conductedwithout enlarging the size of apparatus and the good radiographing canbe performed.

Although the offset correction has been conducted based on the darkcurrent accumulation charge quantity obtained by arithmeticallyoperating the offset data obtained by the radiographing in the secondembodiment, the invention is not limited to such a method. For example,the offset correction can be also conducted by forming the offsetcorrection data by a calculation based on the idling time period, theaccumulation period, and the dark current response characteristicsmeasured upon shipping from the factory or the like.

The continuous motion image radiographing can be also executed byalternately repeating the radiographing of the offset frame and theradiographing (radiation photograph) of the X-ray frame as mentionedabove.

Further, the shot noises can be reduced by performing the radiographingin order of the offset frame radiographing→the X-ray frameradiographing→the offset frame radiographing and conducting the offsetcorrection by the offset frame obtained by averaging the offset framesbefore and after the X-ray frame. In this instance, in addition to thesecond embodiment, the method of the first embodiment is applied, theaccumulation period upon offset frame radiographing which is performedafter the X-ray frame radiographing is adjusted, and the dark currentaccumulation charge quantity of the X-ray frame and each of the darkcurrent accumulation charge quantities of the offset frames before andafter the X-ray frame are equalized, thereby enabling the artifact to bereduced.

Third Embodiment

The third embodiment of the invention will now be described. The firstand second embodiments have been described with respect to the casewhere the radiographing of one X-ray frame and the radiographing of oneoffset frame are performed. In the third embodiment, which will bedescribed hereinbelow, the motion image radiographing is enabled bycontinuously radiographing the X-ray frames.

Since a construction of an imaging system in the third embodiment issimilar to that of the imaging system in the second embodiment, itsdescription is omitted.

An offset correction in the imaging system in the third embodiment willbe described. The operation will be described hereinbelow along a flowof a flowchart illustrated in FIG. 10 with reference to a timing chartillustrated in FIG. 9 and the flowchart illustrated in FIG. 10.

The operation in steps S301 to S307 from the start of the operationuntil the termination of the reading of the offset frame is similar tothe operation in steps S201 to S207 in the second embodiment.

In step S307, as accumulation start time of the first X-ray frame as afirst frame, time txs1 when the reading of the offset frame has beenterminated is supplied to the X-ray frame idling period measuring unit117 and the X-ray frame accumulation period measuring unit 120. TheX-ray frame idling period measuring unit 117 stores the suppliedaccumulation start time txs1 of the first X-ray frame. The X-ray frameidling period measuring unit 117 calculates an idling time period Txi1(=txs1−tis) of the first X-ray frame based on the start time tis of theidling operation and the accumulation start time txs1 of the first X-rayframe and outputs. The X-ray frame accumulation period measuring unit120 stores the supplied accumulation start time txs1 of the first X-rayframe.

After the reading of the offset frame was terminated, the X-ray isirradiated at the first time and the reading of the first X-ray frame isstarted (S308, S309). Upon reading of the first X-ray frame, a quantityof charges output from the photoelectric conversion element 104 is equalto the sum of a dark current accumulation charge quantity Qwx1 of thecharges which are accumulated by the dark current according to the darkcurrent response characteristics and an X-ray charge quantity Qx1obtained by the photoelectric conversion by the X-ray irradiation.

When the reading of the first X-ray frame is terminated, as readingtermination time of the first X-ray frame, the radiographing controlunit 115B supplies time txe1 when the reading has been terminated to theX-ray frame accumulation period measuring unit 120 (S310). The X-rayframe accumulation period measuring unit 120 stores the supplied readingtermination time txe1 of the first X-ray frame. The X-ray frameaccumulation period measuring unit 120 calculates an accumulation periodTx1 of the first X-ray frame based on the accumulation start time txs1and the reading termination time txe1 of the first X-ray frame andoutputs.

Further, the arithmetic operation unit 118 of the X-ray frameaccumulation charge quantity calculates the dark current accumulationcharge quantity Qwx1 of the first X-ray frame by the equation (1) byusing the idling time period Txi1 and the accumulation period Tx1 of theX-ray frame and the dark current response characteristics stored in thedark current response characteristic memory 119.

Subsequently, the image processing unit 110B multiplies the offset imageby (Qwx1/Qw) by using the dark current accumulation charge quantity Qwof the offset frame and the dark current accumulation charge quantityQwx1 of the first X-ray frame, thereby conducting the correctionregarding the dark current component. The image processing unit 110Bconducts the offset correction of the X-ray image of the first frame byusing the dark current corrected offset correction data and subsequentlyexecutes the necessary various types of image processing such as gaincorrection and the like (S311), thereby allowing the corrected image tobe displayed onto the display 121 (S312).

In step S310, the radiographing control unit 115B supplies accumulationstart time txs2 of the second X-ray frame as a second frame to the X-rayframe idling period measuring unit 117 and the X-ray frame accumulationperiod measuring unit 120. The X-ray frame idling period measuring unit117 stores the supplied accumulation start time txs2 of the second X-rayframe. The X-ray frame idling period measuring unit 117 calculates anidling time period Txi2 (=txs2−tis) of the second X-ray frame andoutputs. The X-ray frame accumulation period measuring unit 120 storesthe supplied accumulation start time txs2 of the second X-ray frame.

In the embodiment, after the reading of the X-ray frame was terminated,the radiographing of the next X-ray frame is continuously executed.Therefore, reading termination time txen of the X-ray frame as an nthframe becomes accumulation start time txs(n+1) of the X-ray frame as anext (n+1)th frame.

After the corrected image was displayed onto the display 121, thecontrol unit 114B increases a value of the number n of radiographedframes by “1” (S313) and discriminates whether or not the radiographinghas been terminated (S314).

The operation in steps S308 to S314 is repeated until the termination ofthe radiographing has been determined. While the time is updated, theradiographing of the X-ray frames is performed in order of the secondframe, the third frame, . . . . The arithmetic operation unit 118 of theX-ray frame accumulation charge quantity calculates a dark currentaccumulation charge quantity Qwxn of the X-ray frame of the nth frameevery frame. The image processing unit 110B multiplies the offset imageby (Qwxn/Qw), thereby conducting the correction regarding the darkcurrent component of the offset frame which has been radiographed firstand conducting the offset correction of the X-ray image by using thecorrected offset correction data. In this manner, the image processingunit 110B continuously executes the radiographing of the X-ray framesand executes the radiographing of the motion images.

Other Embodiments of the Invention

The invention also incorporates an example in which in order to makevarious types of devices operative so as to realize the functions of theembodiments mentioned above, a program of software to realize theforegoing functions of the embodiments is supplied to a computer (CPU orMPU) in the apparatus or system connected to the various devices and thevarious devices are made operative according to the program stored inthe computer of the system or apparatus, thereby embodying thosefunctions. In such a case, the program itself of the software realizesthe foregoing functions of the embodiments and the program itselfconstructs the invention. A unit for supplying the program to thecomputer, for example, a storage medium in which such a program has beenstored constructs the invention. As a storage medium for storing theprogram, for example, a flexible disk, a hard disk, an optical disk, amagnetooptic disk, a CD-ROM, a magnetic tape, a non-volatile memorycard, a ROM, or the like can be used. Naturally, even in the case wherethe supplied program realizes the foregoing functions of the embodimentsin cooperation with an operating system, another application software,or the like which is operating in the computer, such a program isincorporated in the embodiment of the invention. Further, naturally, theinvention also incorporates a case where the supplied program is storedin a memory equipped for a function expanding board or a functionexpanding unit regarding the computer and, thereafter, a CPU or the likeequipped for the function expanding board, function expanding unit, orthe like executes a part or all of actual processes based oninstructions of the program. Naturally, a case where the foregoingfunctions of the embodiments are realized by those processes is alsoincorporated in the invention. For example, the invention alsoincorporates a case where the control unit 114A (114B) and the imageprocessing unit 110A (110B) are realized by functions of a computerhaving a CPU, a ROM, and a RAM, a processing program for executing theprocessing operations as mentioned above is preliminarily stored in theROM, and the CPU reads out the processing program from the ROM andexecutes it, thereby making control for realizing the foregoingprocessing operation.

The foregoing embodiments are nothing but specific examples forembodying the invention. A technical scope of the invention must not belimitatively interpreted by those examples. That is, the invention canbe embodied in various forms without departing from a technical idea ofthe invention or its principal feature.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2007-100813, filed Apr. 6, 2007, which is hereby incorporated byreference herein in its entirety.

1. An imaging apparatus comprising: a detection unit including aplurality of conversion elements arranged in an array on a substrate forconverting incident radiation or incident light into an electric signal,to derive an image based on the electric signal; a memory unit forstoring dark current response characteristics of the detection unitbased on a time after applying a bias to the conversion elements; anaccumulation charge quantity arithmetic operation unit for calculating adark current accumulation charge quantity included in the accumulation,based on the dark current response characteristics, a first time periodfrom an applying of a bias to the conversion elements until a start ofan accumulation of the conversion elements for deriving the image and asecond time period from a start of the accumulation until an end of theaccumulation; and an image processing unit for conducting an offsetcorrection of the image derived based on the dark current accumulationcharge quantity.
 2. The imaging apparatus according to claim 1, furthercomprising: an accumulation time period arithmetic operation unit forcalculating a third time period for deriving the dark currentaccumulation charge quantity from the detection unit, based on the darkcurrent response characteristics, the first and second time periods andthe dark current accumulation charge quantity; and a control unit forcontrolling the detection unit according to the third time period,wherein the image processing unit conducts the offset correction of theimage based on offset correction data derived from the detection unitcontrolled by the control unit according to the third time period. 3.The imaging apparatus according to claim 1, wherein the image processingunit conducts the offset correction of the image based on offsetcorrection data derived by an arithmetic operation processing of offsetdata preliminarily derived according to the dark current accumulationcharge quantity of the radiographing calculated from the dark currentresponse characteristics and the first and second time periods.
 4. Theimaging apparatus according to claim 1, wherein the image processingunit conducts the offset correction of the image, based on offsetcorrection data generated based on the first time period, the secondtime period and the dark current response characteristics.
 5. Theimaging apparatus according to claim 1, wherein the conversion elementsare formed from amorphous silicon as a main component material.
 6. Theimaging apparatus according to claim 1, wherein each of the conversionelements comprises a wavelength converter for converting the incidentradiation into light, and a photoelectric conversion element forconverting the converted light into the electric signal.
 7. The imagingapparatus according to claim 1, wherein the imaging apparatus isincorporated in an imaging system that includes a radiation generatingapparatus for generating the radiation.
 8. A controlling method of animaging apparatus that includes a detection unit, which includes aplurality of conversion elements arranged in an array on a substrate forconverting incident radiation or incident light into an electric signal,to derive an image based on the electric signal, and a memory unit forstoring dark current response characteristics of the detection unitbased on a time after applying a bias to the conversion elements, themethod comprising steps of: calculating a dark current accumulationcharge quantity included in the accumulation, based on a first timeperiod from an applying of a bias to the conversion elements until astart of an accumulation of the conversion elements for deriving theimage, a second time period from a start of the accumulation until anend of the accumulation and the dark current response characteristics;and conducting an offset correction of the image derived based on thedark current accumulation charge quantity calculated.
 9. A storagemedium storing a program for a controlling method of an imagingapparatus that includes a detection unit, which includes a plurality ofconversion elements arranged in an array on a substrate for convertingincident radiation or incident light into an electric signal, to derivean image based on the electric signal, and a memory unit for storingdark current response characteristics of the detection unit based on atime after applying a bias to the conversion elements, wherein theprogram is executed by a computer to perform steps of: calculating adark current accumulation charge quantity included in the accumulationbased on a first time period from an applying of a bias to theconversion elements until a start of an accumulation of the conversionelements for deriving the image, a second time period from a start ofthe accumulation until an end of the accumulation and the dark currentresponse characteristics; and conducting an offset correction of theimage derived based on the dark current accumulation charge quantitycalculated.